1. Field of the Invention
This invention relates generally to a system and method for providing high voltage isolation fault detection in a fuel cell hybrid vehicle and, more particularly, to an isolation fault detection system for providing high voltage isolation fault detection in a fuel cell hybrid vehicle, where various voltage potentials are measured and compared with voltage potentials that would occur during an isolation fault to detect the isolation fault.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles generate zero emissions and would be more efficient than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle. A plurality of fuel cells are typically combined in a fuel cell stack to generate the desired output power.
A fuel cell hybrid vehicle includes a fuel cell stack and a secondary power source, such as a battery, where the stack is electrically connected to a fuel cell power bus supply that feeds the electrical drive train of the vehicle and other vehicle systems with electrical power. The output of the battery is connected to a battery power bus, which is electrically coupled to the fuel cell power bus through a DC/DC converter.
In order to provide safe operation of the fuel cell hybrid vehicle, all high voltage parts of the electrical system of the vehicle are electrically isolated from the vehicle chassis. However, various operating conditions of the vehicle may cause a high voltage part of the electrical system to be directly coupled to the vehicle chassis during vehicle operation. For example, the cooling fluid flowing through cooling channels in the stack initially has a low conductivity. However, as time goes by, the cooling fluid becomes contaminated and more conductive, which could cause an impermissible current flow to the vehicle chassis. Additionally, the isolation resistance between the power bus and the vehicle chassis of any component can be reduced by a fault, for example mechanical damage, water inside the component, etc. In the worst case, a short circuit can occur. In the case of a fault, accessible live parts of the power bus from, for example, damaged cable insulation, unplugged connectors, removed converter covers, etc., can create a hazardous current flow through a human body.
The loss of high voltage isolation between a high voltage part of the vehicle electrical system and the vehicle chassis must therefore be detectable during vehicle operation to provide the desired safety. Conventional isolation fault detection devices cannot effectively be used in fuel cell hybrid vehicles because the isolation resistances in the fuel cell stack are asymmetrical. Particularly, because of the stack design and geometry, including the flow of the cooling fluid through the stack, the voltage potential between a positive terminal of the stack and the vehicle chassis ground is different than the voltage potential between the negative terminal of the stack and the chassis ground. Thus, conventional isolation fault detection systems that rely on an imbalance between voltage potentials to indicate the fault cannot be used in a fuel cell system.
When an isolation fault is detected, the isolation fault detection system will take suitable action, such as shutting down the system or providing a warning light to the vehicle operator. Additionally, it is desirable that the isolation fault detection system identify the location of the isolation fault so that it can be readily corrected.